ABSTRACT The overarching hypothesis for the proposed research is that PIC1 peptides can be developed for the pharmacological modulation of myeloperoxidase (MPO)-mediated inflammation in human diseases. MPO is a potent peroxidase that generates hypochlorous acid that can oxidize host tissues contributing to diseases like rheumatoid arthritis and chronic obstructive pulmonary disease. Additionally, MPO has been shown to mediate neutrophil extracellular trap (NET) formation, which is believed to play major roles in systemic lupus erythematosus and glomerulonephritis. Currently no pharmacological intervention is available to inhibit MPO- mediated host tissue damage. PIC1 peptides are a family of 15 amino acid length peptides. The lead compound, PA-dPEG24, more potently inhibits MPO peroxidase activity on a molar basis in vitro than the archetypal MPO-inhibitor 4- aminobenzoic acid hydrazide (ABAH). Although other MPO inhibitors are in development they face a major safety barrier because MPO is vital for neutrophil killing of pathogenic microorganisms. PIC1 peptides are derived from a molecule with defensin homology and have inherent anti-bacterial activity against S. aureus and P. aeruginosa. Thus, PIC1 peptides will be much safer for pharmacological use in humans than other MPO inhibitors currently in development. The proposed studies will address two aims: 1) Molecular analysis of PIC1 inhibition of MPO peroxidase activity and 2) Evaluate PIC1 inhibition of MPO peroxidase activity and inflammation in vivo. The molecular analysis of PIC1 inhibition of MPO will utilize a library of established amino acid substitution derivatives of PIC1 to determine amino acids that are important for binding to MPO as well as inhibition of MPO peroxidase activity. We will also evaluate the ability of PIC1 peptides to inhibit MPO-mediated NET formation. We will then evaluate the extent to which PIC1 can inhibit MPO peroxidase activity in vivo in an established small animal peritonitis model. In these experiments, the dose of PIC1 will be optimized and efficacy will be compared against the archetypal MPO inhibitor ABAH. In addition to testing inhibition of MPO peroxidase activity in the peritoneal lavage fluid, we will also evaluate inflammation and cellular damage to the peritoneal tissues. Successful completion of these studies will provide proof of concept data that PIC1 peptides can effectively inhibit MPO peroxidase activity in vivo. Future studies will focus on refining our mechanistic understanding of PIC1 inhibition of MPO peroxidase activity and testing PIC1 efficacy in animal models of MPO-mediated diseases like rheumatoid arthritis and systemic lupus erythematosis.